The utilization of ceramic fibers for insulation of high temperature furnaces has been practiced in several methods. In one method, sheets of ceramic fiber are attached to the wall much in the manner of layered wallpaper. It is known in such constructions to utilize lower temperature resistant ceramic fibers adjacent the cold face of the shell of the furnace with higher temperature resistant fibers forming the layers of the hot face. This system has disadvantages in that it is difficult to install and requires numerous studs which must be carefully positioned and themselves must be temperature resistant. Studs conduct heat to the cold face thereby decreasing the efficiency of the insulation. Studs are also expensive and subject to breakage from mechanical abuse and/or thermal shock.
It has also been practiced to utilize modules of ceramic fiber which are installed as units with the fibers of the modules arranged such that most of the fibers in the ceramic fiber mats lay in planes generally perpendicular to the wall. U.S. Pat. No. 3,819,468 to Sauder illustrates such a module system. U.S. Pat. No. 3,930,916 also illustrates such a system and refers to the fiber orientation as being "end-on" relative to the supporting layer of the module. These module systems may be installed by screwing, bolting or welding the modules to the furnace shell. These modules have enjoyed success because of the ease of installation, efficiency of heat insulation and ease of replacement of damaged modules. However, known edge grain module systems have suffered from the disadvantage that only one type of ceramic fiber may be utilized in a module. Therefore, expensive high temperature resistant ceramic fibers must be utilized to form the entire module even though the fibers near the cold face are exposed to temperatures much lower than their maximum use temperature.
There are also have been developed very high temperature ceramic fibers formed by solution gelling techniques which are commonly referred to as polycrystalline fibers. Such fibers are illustrated by U.S. Pat. No. 3,996,145 to Hepburn, 3,322,865 to Blaze, Jr., 4,277,269 to Sweeting and 4,159,205 to Miyahara. These polycrystalline fibers have a working temperature up to about 3000.degree. F. However, these fibers are very expensive and expense has limited their use. It also has been a practice in the industry to blend the with lower temperature ceramic fibers to create a higher temperature resistant material with better strength properties and temperature resistance nearly equal to the polycrystalline fibers alone at reduced cost.
It has been proposed to utilize polycrystalline very high temperature resistant fibers in modules either singly or as a blend with lower temperature ceramic fibers. Such modules are ordinarily only about 3 inches in thickness and are cemented with refractory mortar over hard refractories or cemented to ceramic fiber modules. Such installation techniques are expensive in use of fiber and further are subject to failure as the cement bond between the hard refractory and the fibers or between the two types of fiber is subject to failure upon thermal cycling of the furnaces and further is very dependent upon high quality, careful installation.
Modules of composite construction which provide the entire insulation value of a furnace have been proposed. Such modules have a lower temperature fiber forming the interior of a cushion-like block, the outer covering of the cushion being a higher temperature ceramic fiber material. However, it is not believed that the polycrystalline ceramic fiber insulating materials have been successfully used in such constructions as mats of polycrystalline fibers have been too weak when used as a surface covering. They have lacked both strength and abrasion resistance to articles passing into and out of a furnace and even strong air currents in a furnace. Polycrystalline fiber structures of sufficient strength and abrasion resistance have been too rigid to be bent around the sides of the cushion and be compressed during installation. Modules formed of rigid board material are difficult to install without leaving gaps. Further, modules of a construction using several different types of fiber have been found to be expensive in labor costs for construction and in materials costs for fastening of the various elements of such modules together.
Therefore, there remains a need for a system of presenting the very high temperature resistant fibers to the hot face of a furnace wall preferably such that most of the fibers lie in planes generally perpendicular to the furnace wall, using only the minimum amount of these fibers at the hot face. Further, there remains a need for such a system that will allow utilization in module construction or in other constructions which place the edge grain of the module towards the hot face with the fibers of the mat laying in planes generally perpendicular to the wall or ceiling of the furnace. There is a further need for a commercially feasible system of providing insulation to furnaces operating with hot face temperatures of about 3000.degree. F.